1. Field of the Invention
The present invention relates to a clutch control method, and more particularly to a method for controlling the connection/disconnection of a wet friction clutch which is provided in a power transmission system of a vehicle.
2. Description of the Related Art
The present inventors have newly developed a vehicle power transmission device, in which a fluid coupling capable of being locked up (including a torque converter) and a wet friction clutch are serially provided between the engine and transmission, and which automatically connects and disconnects the wet friction clutch during transmission. Here, if the vehicle is put into gear while stationary, the clutch is subsequently automatically connected, and creep is generated thereby. This point is similar to an ordinary AT (Automatic Transmission) vehicle. On the other hand, while the vehicle is traveling after starting off, the fluid coupling is locked up, and the clutch is automatically disconnected/connected at each transmission gear shift. This point is similar to an ordinary MT vehicle.
When connection of the clutch is too rapid, this produces a clutch connection shock (so-called “garage shock” or similar), and when the clutch connection is too slow, it takes time from the gear-in operation until the generation of creep, and the driver does not know when to step on the accelerator (large time lag). Therefore, for the purpose of achieving compatibility between clutch connection shock and a shortening of the connection time, control is performed such that the clutch is rapidly connected in an idle region up until when the clutch begins to connect, and is slowly connected by switching the connection speed once the clutch begins to connect.
More specifically, the working fluid pressure for driving the disconnection/connection of the clutch is changed in accordance with a duty pulse outputted from an electronic control unit, and, when connecting the clutch from a disconnected state, a prescribed start duty for largely connecting the clutch as far as the vicinity of the position in which the clutch begins to connect is initially outputted from the electronic control unit (this will be referred to as a single connection), and, thereafter, a prescribed gradual connection duty for gradually connecting the clutch is outputted from the electronic control unit at prescribed time intervals.
The position in which the clutch begins to connect, i.e. the torque transmission start point at which it is initially possible to transmit a prescribed torque, is known as the “torque point”, and this torque point is utilized, for example, as a connection speed switching point, by making the control unit learn this torque point. This torque point is taken as a learning value since variations or individual differences exist between clutches which are caused by production errors or similar, and the torque point is different for each clutch.
Meanwhile, at the start of development, clutch gradual connection control was open-loop control which outputted a gradual connection duty according to a predetermined schedule. However, as a result of individual differences, such as variations in the torque point learning value, in the clutch characteristics, or in the oil temperature, and the like, and/or deviations in the usage conditions, problems occurred such as the generation of clutch connection shock or a long connection time, which in turn resulted in the inconvenience of an unstable feel.
Meanwhile, with regard to clutch connection control at the time of a gear shift while the vehicle is traveling, at the start of development, shift-up and shift-down clutch connection modes were preset, and such modes were used properly according to whether a shift-up or a shift-down was actually performed.
However, it was established that it is insufficient to simply modify the clutch connection method in terms of only a shift-up or a shift-down. For example, when shifting down from third gear to second gear, it is ordinarily sufficient to connect the clutch in accordance with a shift-down mode following a gear shift to second gear. However, when the clutch is connected by putting the vehicle into neutral from third gear, and then into second gear after coasting a short while until the vehicle speed has dropped sufficiently, there are also times when a better feel is afforded by connecting the clutch in accordance with a shift-up mode instead.
Therefore, a method in which proper usage of clutch connection modes is made using only a judgement of a shift-up or a shift-down is limited in its conformance with a clutch connection feel in all conditions, and, depending on the conditions, there are cases where a clutch connection shock, a connection time lag, and the like, are produced.